This invention relates generally to signal processing in fiber optic gyroscope systems. This invention relates particularly to automatic gain control (AGC) circuits in fiber optic gyroscope signal processing systems. Still more particularly, this invention relates to an AGC circuit that achieves a stable response irrespective of the actual gain level.
A closed-loop fiber optic gyroscope requires a high bandwidth, high performance signal processing scheme to capture the differential phase difference induced by the rotation rate. A description of the control loop is contained in U.S. Pat. No. 5,883,716, which issued to Mark and Tazartes on Mar. 16, 1999 and which is assigned to Litton Systems, Inc., assignee of the present invention. The disclosure of U.S. Pat. No. 5,883,716 is incorporated by reference into the present disclosure. In order to achieve the high degree of performance required while accommodating wide variations in loop gain, an active gain control scheme is required. Loop gain variations are due to aging of the light source, variations in optical output or loss over the operating temperature range, and temperature sensitivity of electro-optic components such as photodetectors. In addition, the optical signal can vary by a significant amount from instrument to instrument due to component and manufacturing tolerances.
Automatic gain control loops have therefore been utilized in the past to ensure that the total loop gain remains constant, which is essential to achieving maximum bandwidth and high order loop response. In the past, analog multipliers were used as gain stages in the detection path of the fiber optic gyroscope. While these devices provided an ideal linear control law (i.e. the gain is directly proportional to the applied control voltage), they exhibited a number of undesirable characteristics. These include bandwidth limitations, noise, cost, and linearity as a function of signal level.
For high performance, low noise fiber optic gyroscopes, an alternate gain control block was therefore considered. This is a variable gain amplifier whose gain in dB is proportional to the applied control voltage. In essence, this implies that the gain of the amplifier is an exponential function of applied control voltage as opposed to a linear function as in the earlier embodiments. Such devices are now readily available and offer lower noise and of course, a wider range of gain adjustment without substantial degradation in signal gain linearity. A gain range of 10:1 is easily achievable with such a device.
The desire to adapt such variable gain amplifiers in a fiber optic gyroscope circuit introduced a new problem which this invention addresses. Because of the non-linear gain control law, the time constant or response time of the AGC (automatic gain control) itself could be highly variable, thus limiting its ability to start-up rapidly and to track changes rapidly.
The present invention overcomes the deficiencies of the prior art by providing a stable AGC response irrespective of the actual gain level.
A closed loop gain control circuit according to the present invention for controlling the gain of a variable gain amplifier that is arranged to amplify electrical signals indicative of optical signal signals output from a fiber optic gyroscope comprises a perturbation injection circuit arranged to provide a perturbation signal xc2x1d. A phase modulator is connected between the perturbation injection circuit and the fiber optic gyro. The phase modulator is arranged to apply the perturbation to the fiber optic gyroscope so that the perturbation signal is superimposed on the gyro output. A variable gain amplifier is arranged to receive the electrical signals indicative of optical signal signals output from the fiber optic gyroscope and provide an amplified signal. A perturbation compensation circuit is arranged to apply perturbation compensation signals to signals output from the variable gain amplifier. The perturbation compensation circuit produces a compensated signal by reducing the magnitude of the perturbation in the amplified signal output from the variable gain amplifier. A gain error circuit is connected to the perturbation compensation circuit. The gain error circuit produces a gain error signal that indicates the magnitude of the perturbation signal remaining in the amplified signal after perturbation compensation. A system processor is connected between the gain error circuit and the variable gain amplifier. The system processor provides a gain control signal to the variable gain amplifier to reduce the magnitude of the gain error signal. Processing circuitry is connected between the perturbation compensation circuit and the phase modulator for determining the rotation rate sensed by the fiber optic gyroscope and for controlling the phase modulator to apply a rate nulling signal to the fiber optic gyroscope.